A major problem with continuous measurements of behavioral and physiological functions in laboratory animal studies has been the inadequacy, or arbitrariness, of the illumination regimens employed. Sudden transitions between darkness and bright light intended to mimic the external day-night cycle, for example, may disrupt circadian functions by denying the organism its natural entrainment signal. We are developing a mathematical algorithm for naturalistic simulation of continuous illumination changes throughout the 24-hour day and across the year and geographical locations, in collaboration with the National Bureau of Standards. The proposed project will apply this algorithm toward design, construction, and testing of a new apparatus for presenting naturalistic illumination changes under controlled laboratory conditions, and manipulating them experimentally. The apparatus will incorporate broad-band fluorescent illumination from starlight through sunlight levels (approximately 0.001 - 4000 lux), as controlled by individual microprocessors with dynamic photodiode feedback, utilizing a system of motor-driven rotating vanes (akin to a venetian blind which can be clamped shut). Behavioral tests will concentrate on circadian ingestive patterns in nocturnal and diurnal species, including an "escape from light" option time-referenced to momentary illumination level. In addition, the apparatus will be made available for testing in circadian neuropharmacology, sleep, and ethology laboratories. If successful, an expanded version of this apparatus will be proposed for testing in human circadian phototherapy, for which the twilight manipulation may enhance bright-light treatment of endogenous depressive disorders.